This disclosure relates generally to an electronic display, for example, a writing/drawing tablet utilizing a pressure sensitive display. In general, Bistable Liquid Crystal Displays (BLCDs), and in particular, Cholesteric Liquid Crystal Displays (ChLCDs), have proven to have great potential to create low cost pressure sensitive displays that are efficient power consumers and that can be utilized in a number of unique devices.
Recently, the Boogie Board® pressure sensitive cholesteric liquid crystal writing tablet, of Improv® Electronics has appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the tablet as described in U.S. Pat. No. 6,104,448. This tablet offers a considerable improvement over previous tablet technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrodes in the tablet. In a cholesteric liquid crystal writing tablet, the liquid crystal is sandwiched between two substrates that are spaced to a particular gap. The upper substrate is flexible and the bottom substrate is painted with a fixed opaque light absorbing dark background. Within the gap is a bistable cholesteric liquid crystal which can exhibit two textures, an essentially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The tablet is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the focal conic state. When one presses on the top substrate with a point stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from essentially transparent to a brilliant reflective color at the location of the stylus. The reflective color contrasts well with the dark background of the lower substrate. An image traced by the stylus or finger will remain on the tablet indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to transparent conducting electrodes on the inner surface of the substrates that drive the cholesteric liquid crystal from its color reflective state back to its essentially transparent state.
The above described principle is disclosed in more detail in U.S. Pat. No. 6,104,448. Polymer dispersions can be used to control the pressure sensitivity and resolution of the image as described in U.S. Patent Application Publication No. 2009/0033811, which is incorporated herein by reference. Other modes of operation and a tablet for multiple color images are described in this patent application publication and a means for select erase is disclosed in U.S. Patent Application Publication No. 2009/0096942, which is incorporated herein by reference and is applicable to the displays of the present disclosure. One mode of operation different from that described above is one in which the tablet is initialized by electrically driving the tablet display to the color reflective texture with a voltage pulse or pulses. Then with a continuous voltage applied to the electrodes of an appropriate value, one can write images by driving the cholesteric material to the substantially transparent texture with the pressure of a pointed stylus. This mode of operation with a color reflective background is termed Mode A whereas the other mode with an essentially transparent background is termed Mode B.
The commercial Boogie Board® writing table, operated in Mode B, has the color black for the fixed opaque light absorbing background. The dark black background offers high contrast for the color reflective image written on the tablet. As disclosed in U.S. Pat. No. 5,493,430, other opaque colors may also be used for the fixed background of a cholesteric liquid crystal display. The color of the background additively mixes with the reflective color to present a different color than that of the cholesteric liquid crystal. There may be multiple colors on the background and those colors may be patterned. As an example, the pattern could be lines offering a lined tablet for convenience in writing text similar to a ruled paper tablet.
A problem with the prior art is that the background color and any background pattern on the tablet are fixed. One cannot change the background pattern; that is in the example of patterned lines, one cannot remove the lines for cases where text is not to be written. We disclose a different electronic display and tablet design whereby the background image can be changed.